Discussions taking place within this forum are intended for the purpose of assisting you in discussing options with your vet. Any other use of advice given here is done so at your risk, is solely your responsibility and not that of this forum or its owner. Before posting it is your responsibility you abide by this Statement
Please always go via the vets for a proper diagnosis and good quality products; mites and fungal can look much more similar than you'd think.
Do not treat in the meantime, as that will postpone any treatment that your vet can administer. make sure that you take all guinea pigs in a group; they all need to be seen to in order to prevent them coming down, too.
A proper mange mite course of treatment consists of 3-4 rounds of either injections or topical treatment, spaced 1-2 weeks apart, depending on the ivermectin product used.